1. Field of the Invention
This invention relates to a level gauge for detecting a level of liquid helium which is accomodated in a container made of metals, glasses or other materials. More particularly, the invention relates to a level gauge for detecting a level of liquid helium which makes use of, as a sensing element, a wire made of an amorphous superconductive alloy obtained by rapid quenching a molten alloy material and in which the level of liquid helium is detected by measurement of an electric resistance of the sensor element.
2. Description of the Prior Art
Masumoto and Inoue disclosed, in Japanese Patent Application No. 57-049911 filed Mar. 26, 1983 and now laid-open in Japanese Laid-open Patent Application Gazette No. 58-167739, level gauges which makes use of wires, made of amorphous superconductive alloys, as sensing elements in order to detect a level of liquid helium.
The amorphous superconductive alloys used to make the sensing elements of the level gauge are alloys which are obtained by rapid quenching molten alloy materials and which comprise 20% vol. or more of an amorphous phase and whose superconducting transition temperature, Tc, ranges from 4.2.degree. to 5.0.degree. K. The alloys are represented by the general formula, Z.sub.a.M.sub.b.(Q+Al).sub.c, in which Z is a member selected from the group consisting of Zr, Hf and Ti, M is a member selected from the group consisting of V, Nb and Ta, Q is Si or Ge, a is a value of 10 to 90 atomic %, b is a value of less than 80 atomic %, and c is a value of 10 to 25 atomic %.
The level gauge disclosed in the above patent application utilizes a wire sensing element of the superconductive alloys. The level gauge is attached to a container in such a way that the sensing element is vertically disposed into liquid helium accomodated in the container. The level of the liquid helium is detected as follows: the sensing element is connected to a D.C. power supply at opposite ends thereof and applied with a small electric current to measure a variation of an electric resistance of the element.
Of the amorphous superconductive alloys represented by the above general formula, Z.sub.a.M.sub.b.(Q+Al).sub.c, an alloy of Zr.sub.80.Nb.sub.5 l.Al.sub.8 Si.sub.7 has the following problems to be solved when applied as a wire sensing element of a level gauge.
(1) Because of the addition of Nb to Zr which is a main ingredient of the superconductive alloy of the above formulation, the melting temperature of the alloy becomes very high, i.e. about 1800.degree. C., so that specific types of equipments or apparatus are needed to melt the alloy. PA0 (2) For the production of the alloy, production conditions such as a melting temperature, a time required for quenching and the like must be determined in detail, resulting in poor workability. PA0 (3) It is very difficult to make a wire having a width below 0.5 mm, which is required as the sensing element of the level gauge. PA0 (4) The superconducting transition temperature Tc, of the alloy is in the range of from 4.2.degree. to 5.0.degree. K. Depending on the production conditions, the temperature may vary within a range of 0.5 to 10%. Accordingly, after the alloy wires are made the individual wire must be measured with respect to the transition temperature which requires an additional selection operation. PA0 (5) When a sensing element of the level gauge is made of an alloy whose superconducting transition temperature, Tc, is about 5.0.degree. K., an electric current of about 50 mA is necessary for the measurement of a level of liquid helium. However, if the current for the measurement is such a high value as indicated above, evaporation of liquid helium is promoted.